1. Field of the Invention
This invention relates generally to the field of conductive polymers.
2. Description of Related Art
Printed electro-active composites are emerging as a useful class of materials to fabricate ultra-low cost disposable consumer electronic devices. The tunable properties and processability of electro-active composite materials make them suitable for use in photovoltaics, transistors, displays, batteries, radio frequency devices, and sensors. Furthermore, the ability of electro-active composite materials to be printed and processed at low temperatures enables printing of components directly on unmodified paper or textiles with minimal impact on function or form factor. Using these printable materials, functional electronic components, including thermochromic displays, cellulose based batteries, antennas and disposable radio frequency identification (RFID) tags have been produced on low cost textiles and paper.
All electronic devices require electrodes to provide power, signal and ground to active and passive components. Printed conductive materials are fundamental to the development of printed electronic devices. Current materials for printed conductors can be stratified into two categories: low temperature sinterable nano-inks and epoxy-based electrically conductive composites (ECC). However, both of these methods to print conductors have their limitations. Sinterable nano-inks have low resistivity (˜2×10−6 to ˜5×10−5 Ωcm), but have insufficient abrasion resistance, adhesion and are typically too expensive for low cost applications. On the other hand, ECCs are relatively lower in cost and have excellent adhesion and abrasion resistance, but have relatively high resistivity (10−4-10−3 Ωcm) at filler loading of 80 wt %. The relatively high resistivity of ECC results from minimal contacts between conductive fillers. This high resistivity of conventional epoxy-based composites makes them inefficient for uses in conventional low powered, high performance or high frequency devices.
Furthermore, the preparation of flexible highly conductive interconnect materials at low temperatures (preferably 150° C. or below) is important for the future of low-cost flexible electronics. The popularity of flexible circuits and building electronic devices on flexible substrates requires the interconnect materials to be mechanically compliant and highly conductive. Low processing temperatures of the interconnect materials are also required to enable the wide use of low cost, flexible substrates such as paper and polyethylene terephthalate (PET). Flexible conductive polydimethylsiloxane (PDMS) composites have been developed for various microelectronic applications, owing to the unique physical and chemical properties of PDMS. These properties include relatively superior elasticity and flexibility, optical transparency, biocompatibility and stable physical and chemical properties over a wide range of temperatures from −50° C. to +200° C.
The resistivity of PDMS filled with 80 wt % bimodal distribution of micron-sized silver flakes is about 7×10−4 Ωcm. A lower point of resistivity of 2×10−4 Ωcm for PDMS filled can be realized with 80 wt % silver particles, but the resistivity levels off even after increased filler loading. This high resistivity of PDMS-based conductive composites translates into large losses and low efficiency, especially at high frequency. Another limitation of flexible conductive PDMS composites lies in the poor adhesion on metal surfaces due to the low surface energy of PDMS. This further limits their wide application as a flexible interconnect material.
The resistivity of a conductive polymer composite is determined by the composite composition (such as filler loading), the surface properties of conductive fillers (such as the presence of a thin layer of lubricant or oxide film on the surface of silver flakes), physiochemical properties of polymer matrix (such as cure shrinkage and the interaction between the polymer matrix and conductive fillers), interlayer thickness, temperature, processing conditions of conductive polymer composites, etc. The resistance of conductive polymer composites is the sum of filler resistances (Rf) and inter-particle contact resistances (Rc). The contact resistance is composed of constriction resistance and tunneling resistance. Constriction resistance occurs because the current must squeeze through the small area of contact. Tunnel resistance is due to an intermediate layer between conductive fillers.
In conductive polymer composites, conductive fillers can be separated by a thin layer of polymer, oxide or lubricant for most commercial silver flakes which have been extensively used for the preparation of highly conductive polymer composites. The thickness of the interface can vary from 10 to 100 Å, depending on the physiochemical properties of the polymer matrix, filler, filler concentration, and the conditions of composite preparation. Relatively low conductivity of conductive polymer composites such as conductive PDMS composites results from physical contact, instead of metallurgical joints, between conductive fillers.
Reducing or even eliminating the contact resistance between conductive fillers is an important aspect for the preparation of highly conductive polymer composites. Therefore, new interconnect materials with low electrical resistivity, good adhesion, flexibility and low processing temperatures are desired for electronic applications.